1. Field of the Invention
The present invention relates to a high frequency amplifier and particularly to a high frequency amplifier with variable capacitors and resistors in its input and output matching circuits, wherein the gain and frequency response can be automatically tuned to avoid the impact of impedance variation of the lead frame or resulting from the manufacturing process.
2. Description of the Prior Art
Wireless communication devices for different purposes, such as GSM or PHS mobile phones, use different transmission bands. Each of them includes a high frequency amplifier in its transmitter and receiver to amplify the signal in the transmission band used.
FIG. 1 is a diagram showing a conventional high frequency amplifier. It includes two transistors M1 and M2, a LC tank composed of an inductor Ld and a capacitor Cd, inductors Ls and Lg, and capacitors Cg and CL. The drain of the transistor M1 is connected to the source of the transistor M2. The gate of the transistor M1 is connected to receive a voltage VDD. The gate of the transistor M2 is connected to the inductor Lg and capacitor Cg connected in series. A signal IN is input to one end of the capacitor Cg. The inductor Ls is connected to the source of the transistor M1. The LC tank is connected to the drain of the transistor M2. The drain of the transistor M2 is connected to the capacitor CL and outputs a signal OUT.
The impedances of the capacitor Cg, inductors Lg and Ls, and the parasitic gate-to-source capacitor (not shown) of the transistor M1 are selected for input impedance matching. Only the signal IN having a resonant frequency formed by the input impedance of the high frequency amplifier is amplified. The gain of the high frequency amplifier is determined by the inductor Ld and capacitor Cd. The inductor Ls with a proper inductance will enhance the linear performance of the amplifier.
However, in a conventional high frequency amplifier, the impedances of the resistors, inductors and capacitors are not tunable after they are determined. Thus, impedance variation of the lead frame or resulting from the manufacturing process has serious impact on the input and output impedance of the high frequency amplifier. It-is difficult to have a conventional high frequency amplifier with a desired precise input and output impedance match.
The object of the present invention is to provide a high frequency amplifier with variable capacitors and resistors in its input and output matching circuits, wherein the gain and frequency response can be automatically tuned to the avoid impact of impedance variation of the lead frame or resulting from the manufacturing process.
The present invention provides a high frequency amplifier comprising a first and second transistor, the first transistor having a drain connected to a source of the second transistor and the second transistor having a gate connected to receive a first voltage, a first, second and third inductor, the first inductor having a first end connected to a gate of the first transistor, the second inductor having a first and second end respectively connected to a source of the first transistor and to receive a second voltage, and the third inductor having a first and second end respectively connected to a drain of the second transistor and to receive the first voltage, a first, second, third, fourth and fifth capacitor, the first capacitor having a first and second end respectively connected to a second end of the first inductor and to receive an input signal, the second capacitor having a first end connected to receive the second voltage, the third capacitor having a first end connected to receive the second voltage, the fourth capacitor having a first and second end respectively connected to the drain of the second transistor and to receive the first voltage, and the fifth capacitor having a first end connected to the drain of the second transistor, a first and second resistor, the first resistor having a first and second end respectively connected to a second end of the second capacitor and to receive the second voltage, and the second resistor having a first and second end respectively connected to a second end of the third capacitor and to receive the second voltage, a first and second variable capacitor, the first variable capacitor having a first and second end respectively connected to a second end of the second capacitor and a second end of the third capacitor, and the second variable capacitor having a first and second end respectively connected to a second end of the fifth capacitor and to output an output signal, a variable resistor having a first and second end respectively connected to the second end of the fifth capacitor and to receive the second voltage, and a received signal strength indicator receiving the output signal and generating an indication signal corresponding to an amplitude of the output signal, by which the first and second variable capacitor, and the variable resistor are tuned.
The present invention further provides a high frequency amplifier comprising a first and second transistor, the first transistor having a drain connected to a source of the second transistor and the second transistor having a gate connected to receive a first voltage, a first, second and third inductor, the first inductor having a first end connected to a gate of the first transistor, the second inductor having a first and second end respectively connected to a source of the first transistor and to receive a second voltage, and the third inductor having a first and second end respectively connected to a drain of the second transistor and to receive the first voltage, a first, second and third capacitor, the first capacitor having a first and second end respectively connected to a second end of the first inductor and to receive an input signal, the second capacitor having a first and second end respectively connected to the drain of the second transistor and to receive the first voltage, the third capacitor having a first end connected to the drain of the second transistor, a first and second variable capacitor, the first variable capacitor having a first and second end respectively connected to a second end of the second capacitor and a second end of the third capacitor, and the second variable capacitor having a first and second end respectively connected to a second end of the third capacitor and to output an output signal, a variable resistor having a first and second end respectively connected to the second end of the third capacitor and to receive the second voltage, and a received signal strength indicator receiving the output signal and generating an indication signal corresponding to an amplitude of the output signal, by which the first and second variable capacitor, and the variable resistor are tuned.
Thus, in the invention, there are varactors and variable resistors in the matching input and output circuits. A received signal strength indicator receives the band-pass filtered intermediate-frequency signal and generates an indication signal corresponding to the amplitude of the received signal. The variable capacitors and resistors are tuned by the indication signal from the received signal strength indicator. Thus, the gain of the high frequency amplifier is automatically controlled.